1. Field of the Invention
This invention relates to so-called sandwich molding in which two kinds of molten resin materials are poured into the molding space of a mold to thereby form a skin layer and a core layer, and in particular to a change-over mechanism and a change-over process whereby discrete resins are poured into the space through a space.
2. Related Background Art
The following conditions are required of a sandwich injection molding machine in which two kinds of resins are successively poured into the molding space of a mold to thereby form a molded article of skin layer resin and the circumference of the core layer can be covered with the skin layer.
(i) That the two kinds of resin materials do not mix together in the molding space during molding.
(ii) Due to the recent development of various business machines and OA (office automation) instruments, high-precision and high-strength plastics are used for the housings and parts of those instruments, and highspeed and high-pressure injection molding is carried out for the improvement of the functions of the parts and therefore, high internal pressure acts on a value mechanism for changing over the resin materials. Therefore, during the repetition of continuous molding, elastic and plastic deformation of the value mechanism occurs due to the repetition stress and comes to affect the accuracy of the molded article and the durability of the molding machine. Particularly, in the case of the sandwich molding, where, for example, a material of relatively low strength and good surface finish is used for the skin layer and a material of high strength (such as fiber-reinforced resin) is used for the core layer, the fluidity of the materials causes the materials to leak from a gap created in the valve mechanism, the leaking materials will damage the valve mechanism and this may adversely affect the molded article.
Therefore, a valve mechanism which will be capable of withstanding high-pressure injection molding is required and also, a mechanism for preventing the valve mechanism from being damaged by the leaking materials is required.
As a sandwich injection molding machine for overcoming the above-noted problems, there is known Japanese Patent Publication No. 28464/1975. A feature of the injection molding machine shown in this patent publication resides in a valve mechanism whereby resin materials from two injection cylinders are poured into a molding space in the mold through a pouring gate passageway, and a feature of this valve mechanism lies in its construction wherein a valve rotating in a predetermined position of the valve body is provided and the valve is provided with a first hole for pouring the resin from a first injection cylinder into said molding space and a second hole for pouring the resin from a second injection cylinder into the same molding space and the valve is rotated so as to open and close the passageways of the first and second holes.
According to the technique of the above-mentioned patent publication, of the above-described conditions required of the sandwich molding, the problem of preventing the resin materials from mixing together can be solved, but the deformation of the valve mechanism caused by the high-pressure injection molding and the problem caused by the leaking resin have not yet been solved.
Also, the construction of the valve of the above-mentioned patent publication is such that discrete holes for resins are provided individually and therefore, alignment between each injection cylinder and the inlet of each hole and alignment between the pouring gate and the outlet of each hole is required, and this leads to a problem such as complicated structure of the valve.